In recent years, hard disk drives have been mounted not only in personal computers but also in domestic electrical products, and there is strong demand for a hard disk drive where the size is smaller than that of the prior arts and where a large capacity recording is possible. In addition, shock resistance has been required in order to mount it inside mobile products such as music players, cellular telephones, and video cameras. In order to achieve large capacity, a perpendicular magnetic recording method is starting to be adopted in place of a conventional longitudinal magnetic recording method. A high density recording state is stable because adjacent magnetizations do not oppose each other in a perpendicular magnetic recording, so that it is considered to be fundamentally suited for high density recording. Moreover, the recording efficiency can be improved by combining a single pole type recording head with a perpendicular magnetic recording medium having a soft-magnetic underlayer. In order to improve the shock resistance, it is important to decrease the impact force by making the head light. However, it is also important to develop a medium with high strength. It is necessary to discuss the material and the combination constituting the medium from the viewpoint of not only the recording/reproducing properties but also the strength of the medium.
A granular type material where oxygen or an oxide is added to a CoCrPt alloy is used for the recording layer of a perpendicular magnetic recording medium. A granular type recording layer includes magnetic grains composed of a CoCrPt alloy and non-magnetic oxide grain boundaries surrounding them. Low noise is achieved by decreasing the exchange coupling between the magnetic grains by using an effect where oxides segregate to the grain boundaries. Therefore, since the formation state of the oxide grain boundaries controls the noise properties, an amount of oxygen and oxide to be included in the recording layer is important.
As a perpendicular magnetic recording medium having a granular type recording layer, for instance, Japanese Patent Publication No. 2003-178413 (“Patent Document I”) discloses a perpendicular magnetic recording medium where the volume of the non-magnetic grain boundaries which are mainly composed of oxide is 15% or more and 40% or less of the volume of the whole magnetic layer. In order to secure the low noise properties by controlling the segregation structure of the granular magnetic layer, it is described that controlling the amount of oxide contained in the magnetic layer is important. Moreover, Japanese Patent Publication No. 2004-310910 (“Patent Document 2”) discloses a granular type perpendicular magnetic recording medium where a magnetic layer which does not include the oxide is laminated over a magnetic layer including the oxide. It is described that compatibility between recording/reproducing properties and resistance to thermal fluctuations is possible because of such a configuration of the magnetic layer. Japanese Patent Publication No. 2005-141825 (“Patent Document 3”) describes a granular type magnetic recording medium where the oxide content in the magnetic layer continuously decreases in the direction from the substrate to the film surface. It is described that noise reduction is possible by achieving such a distribution of the oxide content. Japanese Patent Publication No. 2006-120290 (“Patent Document 4”) describes a technology for improving the recording/reproducing properties while ensuring the flyability and the durability of the head in a granular type perpendicular magnetic recording medium. In this patent document, a medium is proposed, which is characterized by a shape where the diameter of the protective layer side part is greater than the diameter of the intermediate layer side part with regard to the columnar grains in the magnetic recording layer.
With regard to a granular type perpendicular magnetic recording medium, it is reported that the recording/reproducing properties and the resistance to thermal fluctuations are improved by controlling the oxygen content and the oxide, but there has not been enough discussion from the viewpoint of shock resistance.
A contact mark, a so-called scratch, may be formed on the surface of the medium in the hard disk drive when the head rotating at high speed contacts the surface of the medium. In the part of the medium where a scratch is formed, not only has the shape changed but also magnetic information has been lost in many cases or it cannot be written, so that reading becomes impossible. Therefore, in order to obtain a magnetic storage apparatus with high impact resistance and high reliability, it is necessary to develop a magnetic recording medium with excellent durability where it is difficult for a scratch to form if it contacts the head. In order to prevent a magnetic recording medium from forming a scratch and to improve the durability, the protective layer has been improved up to now. However, even if a hard protective layer is used, a scratch is easily formed caused by deformation of the soft layer when a soft layer exists underneath it. Specifically, in order to prevent the formation of a scratch and to improve the durability it is necessary to discuss not only the protective layer, but all layers and combinations thereof included in the medium. Specifically, since a granular type recording layer does not have sufficient strength, there is a concern that it caused a stating point of deformation, but there has not been enough discussion concerning strength up to now.
In order to decrease the medium noise in a perpendicular magnetic recording medium where the recording layer has a granular structure including a plurality of columnar grains and crystal grain boundaries containing the oxide, a means is effective where additions of the oxide forming the grain boundaries of the recording layer is increased and the exchange coupling between the magnetic grains is reduced. However, when such a means is taken, not only is the flyability deteriorated by an increase in the surface roughness of the granular layer, but also formation of a scratch occurs frequently because deformation is easily produced where the granular layer becomes the starting point. On the other hand, when addition of oxide is drastically decreased, although it is expected that the strength of the granular layer increases, it is impossible to obtain the recording/reproducing properties sufficient for high density recording.